Suppressed carrier transmission systems have been used for a variety of reasons, the most prevalent being for the purpose of concentrating of a maximum energy level in the modulating or information carrying part of the transmission signal. This technique has been frequently combined with spread spectrum modulating techniques which have had the further advantages of reducing peak power levels and providing desirable noise rejection characteristics. The digital pattern of pseudo-random noise signals have demonstrated very desirable correlation characteristics and, because of long pattern repetition times, ambiguity problems in long distance ranging systems have been overcome. A common embodiment of these systems utilizes digital pseudo-random noise signals, bi-phase modulated and a suppressed carrier.
A disadvantage of these systems was apparent because the suppressed carrier construction of the modulated transmission signal prevented the use of the carrier as a directly available reference signal for purposes of recovery of the modulation/information signal. Therefore, simultaneous search and recovery methods have been developed to make the carrier available as a reference signal for use in demodulating and recovering of the intelligence from the modulating signal. The prior art systems, therefore, impose a simultaneous two-step requirement for recovery of information; first, search and recovery of the suppressed carrier signal and; second, search and recovery of the modulating signal. The total recovery time is a product function of each of the two steps of the two step process. A reduction in time for recovery is desirable for improvement of system efficiency. Sequential recovery rather than simultaneous recovery would reduce the time required to the sum of the two steps rather than a product thereof and would accomplish the desired significant time reduction. A system capable of modulation recovery without the necessity of suppressed carrier recovery would allow the desired sequential recovery and represent a significant time efficiency.